1. Field of the Invention
The invention relates to a continuously variable transmission (CVT). In particular, the invention relates to the so-called toroidal continuously variable transmission in which a shifting ratio is changed by the movement of power rollers disposed between input disks and output disk.
2. Description of the Related Art
In a toroidal continuously variable transmission, power rollers with a curved outer peripheral surface corresponding to a toroidal surface are generally provided so that the power rollers can be inserted by a pressure of a hydraulic oil between input disks and output disks that have the toroidal surface. In the toroidal continuously variable transmission, a torque is transmitted using a shear force of an oil film of a traction oil that is formed between these input disks, output disks, and power rollers. Further, the power rollers are rotatably supported by trunnions, and the trunnions are swung in the direction along a swinging axis by a hydraulic oil pressure. In this case, the hydraulic oil is supplied by an oil pump that is driven by an engine.
Therefore, where the power rollers supported by the trunnions and the trunnions thereof move together from a neutral position to a shifting position of the input disks and output disks, a tangential force acts between the power rollers and the disks and side slip is generated. Further, the power rollers swing, that is, tilt with respect to the input disks and output disks. As a result, a shifting ratio that is a ratio of rotation speeds of the input disks and output disks is changed.
However, the oil pump that supplies the hydraulic oil is synchronously operated by the engine. Therefore, as long as the engine is driven, the hydraulic oil is supplied from the oil pump and, therefore, the shifting control that inserts the power rollers between the input disks and output disks and causes the trunnions to swing can be executed. However, immediately after the engine is stopped, the supply of hydraulic oil from the oil pump is also stopped. Therefore, the oil pressure of the hydraulic oil decreases gradually and the shifting control is not executed due to the engine stop. In other words, immediately after the engine is stopped, the hydraulic oil pressure does not drop and, therefore, power can be transmitted between the input disks and output disks via the power rollers. However, because an oil pressure sufficient for shifting control cannot be obtained due to a decrease in the hydraulic oil pressure, the trunnions cannot retain the power rollers in a neutral position and the power rollers can move to a shifting position. As a result, where a vehicle moves immediately after the engine is stopped, for example, because the vehicle is towed or travels by inertia, the shifting ratio changes. In this case, for example where the shifting ratio upshifts, a sufficient torque cannot be obtained when the vehicle starts moving and the starting performance of the vehicle can degrade.
Accordingly, a conventional toroidal continuously variable transmission is available in which an output rotation-driven pump that can supply a hydraulic oil by using the vehicle movement is provided in addition to the oil pump (engine-driven pump) (for example, Japanese Patent Application Publication No. 2004-52861 (JP-A-2004-52861)). With such a toroidal continuously variable transmission, the output rotation-driven pump is driven and the hydraulic oil is supplied where the vehicle moves in a state in which the engine is stopped. As a result, the power rollers can be inserted between the input disks and output disks and trunnions can be swung to perform shifting control.
However, in such a conventional toroidal continuously variable transmission, because the output rotation-driven pump that is not used in a normal travel mode is separately provided in addition to the oil pump, the cost is significantly increased. Further, when the engine is stopped and vehicle travels, the trunnions have to be swung and shifting control has to be performed in a state in which the power rollers are inserted between the input disks and output disks. Therefore, the shifting control is performed by electronic control, thereby consuming battery power.